Constant flow rate control valves have numerous applications in piping networks. For example, in a building air conditioning and heating system, water or other liquid at an appropriate control temperature is pumped from a central station through a piping network to various heat exchange units located throughout the building. Some of these heat exchange units are located relatively close to the central station while others are located much farther away. The fluid pressure applied across inputs and outputs of the respective heat exchange units varies widely because of factors such as frictional losses inherent in the flow of the liquid through the piping network and the distances the liquid must travel.
The flow rate in each branch of the network is a direct function of the pressure drop existing across that branch. Two contributors to the existence and magnitude of the pressure drop are line friction and equipment pressure drop. The actual pressure drop of a branch is often different from the original desired or, designed value, leading to a flow rate in that branch which is different from the desired flow rate, which also influences the flow rate of other branches. To obtain the desired flow rate in the various branches of such a network, the network must be hydraulically balanced. Hydraulic balancing often involves adding additional pressure to the system and creating wasteful pressure drops. Pumps are frequently oversized to provide the additional pressure required to balance the network. However, these pumps are operated at other than their optimum performance condition which results in wasteful energy consumption.
Previous constant flow rate control valves have alleviated the need for hydraulic balancing. Such a control valve is disclosed in U.S. Pat. No. 4,766,928 issued to Golestaneh. Golestaneh discloses a constant flow rate control valve including a movable cup member having a plurality of side ports and an orifice on an end wall of the cup member. Pressure differential across the valve moves the cup member against a resilient spring to expose that side port area appropriate for a constant flow rate at that pressure differential. There is always a minimum pressure differential required to establish the desired flow rate for the Golestaneh and other prior art control valves. For some systems, this minimum pressure drop cannot be reached. Another problem with the prior art control valves is that a control range where the flow rate is relatively constant has a maximum pressure differential above which the flow rate is no longer constant. Previous valves have been limited as to the maximum pressure differential across the valve for which a constant flow rate can be maintained.
From the foregoing, it may be appreciated that a need has arisen for a constant flow rate control valve which requires a lower minimum pressure drop than valves in the prior art in order to establish the desired flow rate. Also, a need has arisen to increase the control range of the valve to be able to maintain a constant flow rate at higher pressure differentials than valves found in the prior art.